Endocrine System: Hypothalamus and Pituitary Gland PDF

Copyright EMAP Publishing 2021 This article is not for distribution...

Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Keywords Hypothalamus/Pituitary gland/Endocrine system/Hormones Systems of life Endocrine system This article has been double-blind peer reviewed In this article... E  ndocrine functions of the hypothalamus Hormones of the anterior and posterior pituitary gland Role of pituitary gland hormones in mediating the actions of the hypothalamus Endocrine system 2: hypothalamus and pituitary gland Key points Authors Zubeyde Bayram-Weston is senior lecturer in biomedical science; Maria The hypothalamus Andrade is honorary lecturer in biomedical science; John Knight is associate and pituitary gland professor in biomedical science; all at College of Human and Health Sciences, both lie in the cranial Swansea University. cavity of the skull Abstract The endocrine system consists of glands and tissues that produce and Two major secrete hormones to regulate and coordinate vital bodily functions. This article, the hormones released second in an eight-part series on the endocrine system, explores the anatomy and by the posterior physiology of the hypothalamus and pituitary gland, and how they work together to pituitary gland are regulate and coordinate vital physiological processes in the body through hormonal antidiuretic action. It shows how many of the actions initiated by the hypothalamus are mediated hormone and through hormonal secretions produced by the pituitary gland beneath it. oxytocin Citation Bayram-Weston Z et al (2021) Endocrine system 2: hypothalamus and The anterior pituitary gland. Nursing Times [online]; 117: 6, 49-53. pituitary gland T produces several stimulating he first article in this eight-part system is well developed in all higher ver- hormones that series on the endocrine system tebrates and plays a key role in emotional regulate the activity gave an overview of the nature of responses, long-term memory, sense of of other endocrine endocrine glands and high- smell (olfaction) and the acquisition of glands lighted the role of hormones as chemical new skills, as well as contributing to a signals that help maintain the homeostatic range of behavioural responses (Wróbel, Although the balance essential to health; the remaining 2018). The hypothalamus is the location of pituitary gland is articles will each explore different major the thermoregulatory centre, which regu- often referred to as endocrine glands and tissues. This article lates body temperature (VanPutte et al, the master gland, examines the anatomy and physiology of 2017); it plays an essential role in water bal- many of its actions the hypothalamus and pituitary gland, ance, regulation of blood pressure and the are directed by the which lie in the cranial cavity of the skull. sensations of thirst and hunger. hypothalamus This article focuses on the endocrine The hypothalamus functions of the hypothalamus and its role Clusters of neurons The hypothalamus is located at the base of as the key link between the nervous and in the hypothalamus the brain just below the thalamus. A small endocrine systems. The hypothalamus is synthesise releasing but vital region of the brain, it is roughly connected directly to the pituitary gland and inhibiting the size of an almond and weighs around via a thin stalk, called the infundibulum hormones that 4g (Saper and Lowell, 2014). It accounts for (Fig 1). Many actions initiated by the hypo- regulate the activity 2.4m. Elevated secretion of mones that regulate the activity of many known as an anabolic hormone. GH in adults, after their epiphyseal growth other endocrine glands is why the pitui- Thyroid hormones T3 and T4 (thy- plates have fused, can lead to acromegaly, tary is often referred to as the ‘master roxine), which regulate metabolism, are in which the hands, feet and some facial gland’. As explained below, this is a mis- necessary for GH to exert its effects effi- features (particularly the lower jawbone) nomer: the release of these stimulating ciently. The anabolic effects of GH are also can grow abnormally large and usually out hormones is governed by hormones enhanced by the presence of other anabolic of normal proportion (de Herder, 2009). released from the hypothalamus, which hormones such as testosterone. As well as ultimately acts as the true primary orches- promoting bone and muscle growth, GH Prolactin trator of endocrine function. also stimulates the growth of many of the Prolactin (lactogenic hormone) initiates The anterior pituitary’s cells are usually major internal organs (Devesa et al, 2016). milk secretion (lactation) in breast tissue. By classified into five major types based on the GH secretion is regulated by two hor- itself, prolactin has only a weak effect, but nature of their secretions. These are listed mones produced by the hypothalamus: during pregnancy prolactin levels increase below with their hormonal secretions: l G  rowth hormone-releasing hormone and it acts synergistically with other hor- l S omatotrophs – somatotropin or stimulates the release of GH; mones – including oestrogens, proges- growth hormone (GH); l G  rowth hormone-inhibiting hormone terone and cortisol – to promote the enlarge- l L actotrophs – prolactin; (GHIH) acts antagonistically to inhibit ment and engorgement of the breasts in l T hyrotrophs – thyroid-stimulating the release of GH (Table 1). preparation for lactation (Suarez et al, 2015). hormone (TSH); Deficiency of GH during childhood It has been hypothesised that the release l C orticotropths – adrenocorticotropic may result in pituitary dwarfism; this is of prolactin is regulated and fine-tuned by hormone (ACTH) and melanocyte- characterised by below-average growth the antagonistic actions of a prolactin- JENNIFER N. R. SMITH stimulating hormone (MSH); and, commonly, an underdeveloped releasing hormone and a prolactin-inhib- l G onadotrophs – follicle-stimulating bridge of the nose and prominent fore- iting hormone, both of which are thought hormone (FSH) and luteinising head. Unlike achondroplastic dwarfism (a to be produced by the hypothalamus hormone (LH). genetic disorder), pituitary dwarfism, (Table 1). Nursing Times [online] June 2021 / Vol 117 Issue 6 51 www.nursingtimes.net  Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life Table 1. Major hypothalamic releasing and inhibiting hormones Hypothalamic hormone secreted Target anterior Anterior pituitary gland Physiological result pituitary cells secretion/release Growth hormone-releasing hormone Somatotrophs Growth hormone Stimulates growth Growth hormone-inhibiting Somatotrophs Inhibits growth hormone Inhibits growth hormone secretion Prolactin-releasing hormone Lactotrophs Prolactin Thought to stimulate lactation (hypothetical) Prolactin-inhibiting hormone Lactotrophs Inhibits prolactin secretion Thought to inhibit lactation (hypothetical) Thyrotropin-releasing hormone Thyrotrophs Thyroid-stimulating Stimulates release of T3 and T4 (thyroxine), hormone thereby increasing metabolism Corticotropin-releasing hormone Corticotrophs Adrenocorticotropic Regulates glucocoticoids, mineralocorticoid hormone and androgen secretion by the adrenal cortex Gonadothropin-releasing hormone Gonadotrophs Luteinising hormone Stimulates ovulation/progesterone secretion in females and testosterone secretion in males Gonadothropin-releasing hormone Gonadotrophs Follicle-stimulating hormone Stimulates maturation of ovarian follicles in females and spermatogenesis in males Tropic hormones cholesterol. It is referred to as a glucocorti- Melanocyte-stimulating hormone Tropic hormones have a stimulating effect coid because it is produced by the adrenal MSH is synthesised by the pars intermedia on other endocrine glands, inducing the cortex and influences the concentration of region of the pituitary gland. Although synthesis and secretion of the target glucose in the blood (VanPutte et al, 2017). this region marks the boundary where the hormone(s). Four major tropic hormones Following periods of chronic stress anterior and posterior portions of the pitu- are synthesised and secreted by the ante- (including classic biological stressors such itary gland fuse, it is generally considered rior pituitary, as described below. as starvation or physical injury), the hypo- part of the anterior pituitary. The pars thalamus releases corticotropin-releasing intermedia atrophies (shrinks) with age Thyroid-stimulating hormone (thyrotrophin) hormone. This initiates the release of and, in adults, may only be present as a TSH stimulates the thyroid gland to secrete ACTH from the anterior pituitary and, vestigial remnant or, in some cases, is not the iodine-containing hormones T3 and T4. subsequently, stimulates the release of cor- recognisable at all. MSH exists in a range of These are primarily responsible for regu- tisol from the adrenal cortex (Table 1). structurally similar forms known as mel- lating metabolism, with T3 being the more Cortisol plays a key role in regulating anocortins, which are all small peptides. potent. Most cell types in the body have metabolism and, during periods of food As implied by its name, MSH stimulates internal receptors for T3 and T4. These hor- deprivation, stimulates the breakdown of the pigment-producing cells (melanocytes) mones are also vital for growth and devel- protein and fat to generate glucose for use in the epidermis to release the dark pig- opment, and play key roles in the normal as fuel in glucose-dependent tissues, such ment known as melanin, which is largely functioning of the cardiovascular, respira- as the brain. This process is called glucone- responsible for skin colour. All races are tory, skeletal and central nervous systems. ogenesis (literally, the creation of new glu- thought to have similar numbers of mel- The release of TSH is regulated by thy- cose). Cortisol also influences the sleep/ anocytes in their epidermis; it is the rela- rotropin-releasing hormone, which is pro- wake cycle, mood and behaviour, and has tive activity of these cells and the amount duced by the hypothalamus (Table 1). The potent anti-inflammatory/immunosupres- of melanin they synthesise and release that fine tuning of T3 and T4 release is regu- sant properties (Kandhalu, 2013). ultimately determines skin colour. lated by negative feedback, through the ACTH also helps to regulate the release Melanocytes can synthesise MSH when sequential secretions of the hypothal- of other steroid hormones produced by the exposed to the ultraviolet (UV) light in amus, anterior pituitary and thyroid gland adrenal cortex, including aldosterone sunlight (Tsatmali et al, 2002). This is (Fitzgerald and Bean, 2018). This hormonal (which regulates the concentration of essential to protect the actively dividing cascade is referred to as the hypothalamic- sodium and potassium in the blood) and cells of the epidermis from the harmful pituitary-thyroid (HPT) axis and will be the group of testosterone-like hormones effects of UV, known to cause DNA damage explored in detail in part 3 of this series. known as androgens (Gallo-Payet, 2016). that can lead to mutations and, potentially, The complex interplay between the hypo- skin cancers. Melanin is excellent at Adrenocorticotrophic hormone thalamus, anterior pituitary and the absorbing UV wavelengths of light and, as (adrenocorticotropin) adrenal cortex is referred to as the HPT axis it accumulates in the epidermis the skin, ACTH primarily regulates the production and will be examined in detail in part 4 of darkens and develops a protective suntan. and secretion of cortisol from the adrenal this series. During pregnancy, levels of MSH tend cortex (outer portion of the adrenal gland). ACTH is also part of the melanocortin to increase, which, together with changes Cortisol is a long-term stress hormone and group of hormones, which influence skin to the sex hormones oestrogen and proges- a steroidal hormone synthesised from pigmentation (see below). terone, often leads to hyper-pigmentation Nursing Times [online] June 2021 / Vol 117 Issue 6 52 www.nursingtimes.net  Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life around the eye sockets, cheekbones, lips of the larynx (causing the deepening of the Costin G-E, Birlea S-A (2006) What is the mechanism for melasma that so commonly and forehead. This is known as melasma or voice) and spermatogenesis, and is largely accompanies human pregnancy? International ‘the mask of pregnancy’; these pigmented responsible for the male sex drive. Union of Biochemistry and Molecular Biology Life; areas usually fade gradually after child- The role of the gonadotropins and male 58: 1, 55-57. De Herder WW (2009) Acromegaly and birth (Costin and Birlea, 2006). and female sex hormones will be discussed gigantism in the medical literature. Case ACTH (described above) is another hor- further in part 7 of this series. descriptions in the era before and the early years mone that can influence skin pigmenta- after the initial publication of Pierre Marie (1886). Pituitary; 12: 3, 236-244. tion through the direct stimulation of mel- Role of the hypothalamus De Sousa SMC et al (2015) Pituitary hyperplasia: anocytes. This is particularly true of The pituitary gland is often referred to as case series and literature review of an under- recognised and heterogeneous condition. certain forms of Cushing’s syndrome, in the master gland but, in fact, it plays more Endocrinology, Diabetes and Metabolism Case which excess ACTH often causes regions of a ‘middle-management’ role; many of its Reports; 2015: 150017. of dark, hyperpigmented skin; this will be actions are directed by the hypothalamus. Devesa J et al (2016) Multiple effects of growth hormone in the body: is it really the hormone for discussed further in part 4 of this series. growth? Clinical Medicine Insights. Endocrinology Hypothalamic nuclei and and Diabetes; 9: 47-71. Gonadotropins hypothalamic-pituitary portal system Fitzgerald SP, Bean NG (2018) Thyroid stimulating hormone (TSH) autoregulation reduces variation in These act on the gonads (testes and ova- The hypothalamus contains discrete, the TSH response to thyroid hormones. ries) to stimulate the production of sex organised clusters of neurons called the Temperature; 5: 4, 380-389. Gallo-Payet N (2016) Adrenal and extra-adrenal hormones and sperm or ova in males and hypothalamic nuclei, which synthesise the functions of ACTH. Journal of Molecular females respectively (see below). The main hypothalamic releasing and inhibiting Endocrinology; 56: 4, T135-T156. gonadrotrophins are FSH and LH; the hormones that regulate the activity of the Ilahi S, Ilahi TB (2020) Anatomy, Adenohypophysis (Pars Anterior, Anterior Pituitary). StatPearls release of both is regulated by gonado- anterior pituitary. Both the hypothalamus Publishing. tropin-releasing hormone, which is pro- and pituitary gland are highly vascularised Kalra S et al (2016) Diabetes insipidus: the other diabetes. Indian Journal of Endocrinology and duced by the hypothalamus (Table 1). and have a dedicated network of blood ves- Metabolism; 20: 1, 9-21. In females, each month FSH initiates sels called the hypothalamic-pituitary Kanbay M et al (2019) Antidiuretic hormone and the development of immature follicles in portal system, which ensures rapid and serum osmolarity physiology and related outcomes: what is old, what is new, and what is the ovaries. As each follicle enlarges, it efficient delivery of the releasing and unknown? Journal of Clinical Endocrinology and secretes the female sex hormone oestrogen, inhibiting hormones from the hypothal- Metabolism; 104: 11, 5406-5420. before maturing into a Graafian follicle, a amus to the anterior pituitary below (Bear Kandhalu P (2013) Effects of cortisol on physical and psychological aspects of the body and fluid-filled, pressurised sac containing a et al, 2021). effective ways by which one can reduce stress. mature ovum (egg), primed and ready to Berkeley Scientific Journal; 18: 1, 14-16 Osilla EV, Sharma S (2020) Oxytocin. StatPearls rupture. Ovulation is triggered by LH, Release of hypothalamic hormones Publishing. which initiates rupturing of the follicle and Secretion of the hypothalamic releasing Parmar P, Malik S (2017) Oxytocin: the hormone ovarian wall; this explosive event propels and inhibiting hormones is determined by of love. International Organization of Scientific Research Journal of Pharmacy and Biological the ovum into its adjacent fallopian tube. multiple sensory inputs, which continu- Sciences; 12: 6, 1-9. Following ovulation, the remnants of the ally monitor the changing physiological Pereira Suarez AL et al (2015) Prolactin in inflammatory response. Advances in Experimental Graafian follicle collapse to form a structure status of the body. Multiple parameters Medicine and Biology; 846: 243-264. known as the corpus luteum (yellow body). monitored continuously and in real time The Pituitary Foundation (2016) Diabetes This produces the second major female sex include temperature, pH, solute concen- Insipidus. The Pituitary Foundation. Rose SR et al (2014) Growth hormone therapy hormone, progesterone, which maintains trations and current levels of circulating guidelines: clinical and managed care perspectives. the integrity of the endometrial lining of hormones. The hypothalamus functions American Journal of Pharmacy Benefits; 6: 5, the uterus to allow for the implantation of a as the key bridge between the nervous and e134-e146. Saper CB, Lowell BB (2014) The hypothalamus. fertilised ovum (VanPutte et al, 2017). endocrine systems, but many of the inter- Current Biology; 24: 23, R1111-1116. Despite their names being reflective of actions between the two remain poorly Tsatmali M et al (2002) Melanocyte function and its control by melanocortin peptides. Journal of the role played in the female ovarian cycle, understood. Histochemistry and Cytochemistry; 50: 2, 125-133. FSH and LH also play crucial roles in male Table 1 summarises the key hormones of VanPutte CL et al (2017) Seeley’s Anatomy and reproductive physiology. FSH is essential the hypothalamus and pituitary, and their Physiology. McGraw-Hill. Wróbel G (2018) The structure of the brain and in stimulating spermatogenesis, where relationships. Some of the better-studied human behaviour. Pedagogy and Psychology of diploid cells (containing 46 chromosomes) interactions between the hypothalamus, Sport; 4: 1, 37-51. undergo meiotic division to produce vast pituitary and peripheral endocrine glands numbers of haploid spermatozoa (each (such as the HPT axis and hypothalamic- CLINICAL containing 23 chromosomes). pituitary-adrenal axis) will be explored SERIES Endocrine system FSH also stimulates the activity of Ser- later in this series. Part 3 focuses on the thy- Part 1: Overview of the endocrine system toli cells (‘nurse’ cells) in the testes; these roid and parathyroid glands. NT and hormones May provide nutrition to the developing sper- Part 2: Hypothalamus and pituitary gland Jun matozoa, allowing maturation into viable References Part 3: Thyroid and parathyroid glands Jul Babu R et al (2013) Symptomatic Rathke’s cleft gametes that are capable of fertilisation. cyst with a co-existing pituitary tumor; brief review Part 4: Adrenal glands Aug LH stimulates the interstitial cells (Leydig of the literature. Asian Journal of Neurosurgery; 8: Part 5: Pineal and thymus glands Sep 4, 183-187. cells) of the testes to synthesise and release Babu SR et al (2004) Evaluation of FSH, LH Part 6: Pancreas, stomach, liver, small the male sex hormone testosterone (Babu intestine Oct and testosterone levels in different subgroups et al, 2004). This powerful anabolic steroid of infertile males. Indian Journal of Clinical Part 7: Ovaries and testes, placenta Biochemistry; 19: 1, 45-49. (pregnancy) Nov stimulates skeletal muscle development, Bear MH et al (2021) Neuroanatomy, Part 8: Kidneys, heart and skin Dec growth of facial and body hair, expansion Hypothalamus. StatPearls Publishing. Nursing Times [online] June 2021 / Vol 117 Issue 6 53 www.nursingtimes.net